This invention relates to specimen holders for use in electron instruments such as, for example, electron microscopes and electron diffraction cameras.
To view or photograph specimens in electron microscopes or electron diffraction cameras it is frequently desirable to tilt the specimen by use of very small manipulating apparatus located within the microscope or camera. It is also desirable for some purposes to vary the temperature of the specimen and for this purpose it has in the past been proposed to thermally couple a holder for the specimen to a bath of liquid nitrogen to reduce the specimen holder's temperature and to locate the specimen on a grid forming part of the specimen holder which can be heated by passing an electrical current through the grid.
In Australian Patent Specification No. 291,412 there is disclosed a mechanism for supporting and tilting a specimen in an electron microscope or diffraction cameras. The mechanism comprises a specimen holder supported at its periphery by three arms, two of which depend from opposite sides of the holder. The specimen holder is supported above a hollow conical stem with the arms extending down the outer periphery of the stem. Actuation means is provided to cause vertical movement of the arms to cause the specimen holder to pivot about a line extending through the two opposite arms at their point of contact with the periphery of the specimen holder. By actuation of each of the arms the specimen holder can be tilted in any direction.
The specimen holder disclosed in Australian Pat. No. 291,412 comprises a milled block in two body parts constructed of thermally and electrically conductive material such as brass and secured so that they are insulated from one another. A fine mesh grid is arranged to extend across the space defined by the arms of the `U` of the base part, being wedged in a dovetail formed in the body parts of the holder. The specimen is positioned on this grid. At three positions on the periphery of the holder are provided outwardly extending conical lugs which are arranged to be located in apertures provided at the ends of the manipulator arms. Two of the arms are arranged to be coupled to a source of electrical current so that this current can be passed through the arms and through the body portions of the holder so that the grid couples the electrical circuit. By passing a current through the grid it is possible to achieve for instance, temperatures sufficient to melt grids made from palladium or platinum whilst the surrounding temperature is maintained at liquid nitrogen temperature by thermally coupling the holder to a source of liquid nitrogen. The specimen holder is normally covered by an anti-contamination cap having a small aperture therein to allow the passage of an electron beam.
In conventional electron microscopes or diffraction cameras, the objective lens pole piece is of cylindrical symmetry with a central bore of very small diameter normally of the order of 6 mm. It is essential to incorporate the tilting mechanism within the focal plane of this pole piece so that the specimen can be tilted without sacrificing the resolution of the microscope. Accordingly space within the pole piece is at a premium and the tilting mechanism must be of conforming dimensions.
A disadvantage encountered with the specimen holder of the above patent is that, because the holder is milled in two parts from a block of brass, the specimen holder is comparatively thick and bulky which reduces the allowable angle of tilt and also restricts the visibility of the specimen. Furthermore, the construction is very complicated to assemble due to its minute size and in particular, positioning of the projecting conical lugs on the periphery of the body portions is extremely complicated and requires accurate assembly on a mandrel. This assembly has to be very accurate since two of the conical lugs determine the axis about which the specimen holder pivots. A further disadvantage with this type of specimen holder is the location of the grid. Location of the grid within a dovetail by wedging or clamping tends to introduce strains in the grid which can introduce an undesirable image drift or, in certain circumstances, can cause the grid to fail. Clamping the grid by the use of screws necessarily increases the depth of the block in which the screws are located, thus undesirably decreasing the allowable tilt angle due to obstruction as mentioned above and increasing the electrical resistance of contact with the grid. The specimen holder has to be frequently cleaned, normally by ultrasonic techniques.
It is with these problems and disadvantages in mind that the present invention has been devised.